Method and apparatus for treating a hydrocarbon stream and method of cooling a hydrocarbon stream

ABSTRACT

Method and apparatus for treating a mixed hydrocarbon feed stream ( 10 ), such as a natural gas stream. The mixed hydrocarbon feed stream ( 10 ) is expanded into a mixed-phase hydrocarbon stream ( 20 ), which is separated the to provide at least a light overhead stream ( 30 ) and a heavy bottom stream ( 50 ). The light overhead stream ( 30 ) is compressed in one or more first compressors ( 16 ) to provide one or more first compressed light streams ( 40 ) and further compressed in one or more second compressors ( 22 ) to provide one or more further compressed light streams ( 60 ). One or more first compressor recycle lines ( 42 ) extend around the or each first compressor ( 16 ), and one or more second compressor recycle lines ( 32 ) having one or more in-line coolers ( 34 ) extend around the or each second compressor ( 22 ). The method and apparatus may be used in a method of cooling an initial hydrocarbon stream ( 100 ).

The present invention relates to a method and apparatus for treating amixed hydrocarbon stream. In another aspect, the present inventionrelates to a method of cooling an initial hydrocarbon stream.

A common example of a mixed hydrocarbon stream is natural gas, whichoften consists of multiple components.

Natural gas is a useful fuel source, as well as being a source ofvarious hydrocarbon compounds. It is often desirable to liquefy naturalgas in a liquefied natural gas (LNG) plant at or near the source of anatural gas stream for a number of reasons. As an example, natural gascan be stored and transported over long distances more readily as aliquid than in gaseous form because it occupies a small volume and doesnot need to be stored at high pressure.

Usually, natural gas, comprising predominantly methane, enters an LNGplant at elevated pressures and is pre-treated to produce a purifiedfeed stream suitable for liquefaction at cryogenic temperatures. Thepurified gas is processed through a plurality of cooling stages usingheat exchangers to progressively reduce its temperature untilliquefaction is achieved. The liquid natural gas is then further cooledand expanded to final atmospheric pressure suitable for storage andtransportation.

In addition to methane, natural gas usually includes some heavierhydrocarbons and impurities, including but not limited to carbondioxide, sulphur, hydrogen sulphide and other sulphur compounds,nitrogen, helium, water and other non-hydrocarbon acid gases, ethane,propane, butanes, C₅+ hydrocarbons and aromatic hydrocarbons. These andany other common or known heavier hydrocarbons and impurities eitherprevent or hinder the usual known methods of liquefying the methane,especially the most efficient methods of liquefying methane. Most if notall known or proposed methods of liquefying hydrocarbons, especiallyliquefying natural gas, are based on reducing as far as possible thelevels of at least most of the heavier hydrocarbons and impurities priorto the liquefying process.

Hydrocarbons heavier than methane and usually ethane are typicallycondensed and recovered as natural gas liquids (NGLs) from a natural gasstream. The NGLs are usually fractionated to yield valuable hydrocarbonproducts, either as products steams per se or for use in liquefaction,for example as a component of a refrigerant.

Meanwhile, methane recovered from the NGL recovery is usuallyrecompressed for use or reuse either in the liquefaction, such as a fuelgas, or being recombined with the main methane stream being liquefied,or it can be provided as a separate stream.

EP 1 031 803 A2 describes a method and apparatus for maximising theproduction rate of NGL in a gas processing plant. Natural gas passesthrough a turboexpander, a recompressor and a booster compressor, eachhaving an antisurge valve and a cold recycle valve. If an operatingpoint of either compressor achieves a set point of its CRIC controlleror of the two UIC controllers, specific signals open the cold recyclevalve first.

A problem with EP 1 031 803 A2 is that cold recycle of a fullycompressed stream around the recompressor will also affect the pressurein the stream it joins from the separator, which will affect thepressure in the separator itself, changing the operation of theseparator and thus its separation efficiency.

In a first aspect, the present invention provides an apparatus fortreating a mixed hydrocarbon feed stream, the apparatus at leastcomprising an NGL recovery system comprising:

-   (a) an expander to expand the mixed hydrocarbon feed stream into a    mixed-phase hydrocarbon stream;-   (b) a first gas/liquid separator to separate the mixed-phase    hydrocarbon stream to provide at least a light overhead stream and a    heavy bottom stream;-   (c) a first compressor having a first inlet and a first outlet, to    compress the light overhead stream and to provide a first compressed    light stream;-   (d) a second compressor having a second inlet and a second outlet,    to compress the first compressed light stream to provide a further    compressed light stream;-   (e) a first compressor recycle line around the first compressor    between the first outlet and the first inlet; and-   (f) a second compressor recycle line provided with one or more    in-line coolers around the second compressor between the second    outlet and the second inlet.

This apparatus may be comprised in an apparatus, such as a plant orfacility, for liquefying a hydrocarbon stream, such as natural gas. Sucha liquefying apparatus may further comprise at least one or more coolingstages upstream and/or downstream of the NGL recovery system. In aparticular embodiment, the liquefying apparatus may further comprise

-   -   a first cooling stage of the one or more cooling stages, able to        cool an initial hydrocarbon stream to provide a cooled and        partly condensed initial hydrocarbon stream;    -   a separator able to separate the cooled and partly condensed        initial hydrocarbon stream to provide the mixed hydrocarbon feed        stream and a methane enriched overhead stream; and    -   a second cooling stage of the one or more cooling stages, able        to cool the overhead stream to provide a combined liquefied        hydrocarbon stream.

In a second aspect, the present invention also provides a method oftreating a mixed hydrocarbon feed stream, the method at least comprisingthe steps of:

-   (a) providing a mixed hydrocarbon feed stream;-   (b) expanding the mixed hydrocarbon feed stream through an expander    to provide a mixed-phase hydrocarbon stream;-   (c) passing the mixed-phase hydrocarbon stream into a first    gas/liquid separator to provide at least a light overhead stream and    a heavy bottom stream;-   (d) passing the light overhead stream through a first compressor    having a first inlet and a first outlet, to provide a first    compressed light stream;-   (e) passing the first compressed light stream through a second    compressor having a second inlet and a second outlet, to provide a    further compressed light stream; and-   (f) selectively recycling at least a fraction of the first    compressed light stream through a first compressor recycle line,    around the first compressor between the first outlet and the first    inlet; and-   (g) selectively cooling and recycling at least a fraction of the    further compressed light stream through a second compressor recycle    around the second compressor between the second outlet and the    second inlet, the second compressor recycle line provided with one    or more in-line coolers.

In a third aspect, the present invention also provides a method ofcooling, preferably liquefying, an initial hydrocarbon stream, such as anatural gas stream, comprising at least the steps of:

-   (i) passing the initial hydrocarbon stream through a separator to    provide a stabilized condensate stream and a mixed hydrocarbon feed    stream;-   (ii) treating the mixed hydrocarbon feed stream by a method    according to the third aspect of the invention; and-   (iii) cooling, preferably liquefying, at least a fraction of the one    or more further compressed light streams to provide a cooled,    preferably liquefied, hydrocarbon stream.

Embodiments and examples of the present invention will now be describedby way of example only with reference to the accompanying non-limiteddrawings in which;

FIG. 1 is a diagrammatic scheme for a first apparatus and method fortreating a mixed hydrocarbon stream according to one embodiment of thepresent invention;

FIG. 2 is a diagrammatic scheme of a method of cooling an initialhydrocarbon stream including embodiments shown in FIG. 1; and

FIG. 3 is a diagrammatic scheme for a second apparatus and method forcooling an initial hydrocarbon stream including treating a mixedhydrocarbon stream according to a second embodiment of the presentinvention.

For the purpose of this description, a single reference number will beassigned to a line as well as a stream carried in that line.

The methods and apparatus disclosed herein may form part of or be usedin a multi-column natural gas liquids (NGL) recovery system andarrangement. An apparatus for treating a mixed hydrocarbon feed streamwherein recompression of the light overhead stream from the firstgas/liquid separator is carried out by one or more first compressors andone or more second compressors, each of the compressors having aseparate recycle line therearound, is provided.

An advantage of this arrangement is a simplified apparatus andsimplified and easier individual compressor control compared with thatshown in EP 1 031 803 A2.

A second advantage of the apparatus disclosed herein is that the or eachrecycle line around the or each first compressor does not need to becooled by a cooler, such as an expensive water and/or air cooler, as theor each first recycle line is dedicated to the or each first compressor.Thus, only the recycle stream around the second compressor requirescooling, significantly reducing the cooling duty required compared tothe cooling duty required for the complete recompressed stream in EP 1031 803 A2. This therefore significantly reduces the CAPEX and OPEXrequired for cooling only a recycle stream.

By ‘recycle line’ is meant a branch line from downstream of the or eachoutlet of the one or more first or second compressors which is connectedupstream from the or each inlet of the one or more first or secondcompressors respectively.

For example, the one or more first compressor recycle lines may have oneor more first recycle line outlets upstream of the one or more firstinlets of the one or more first compressors and one or more firstrecycle line inlets downstream of the one or more first outlets of theone or more first compressors. As a further example, the one or moresecond compressor recycle lines may have one or more second recycle lineoutlets upstream of the one or more second inlets of the one or moresecond compressors and one or more second recycle line inlets downstreamof the one or more second outlets of the one or more second compressors.More preferably the one or more second recycle line outlets liedownstream of the one or more first recycle line inlets.

Referring to the drawings, FIG. 1 shows an apparatus and method fortreating a mixed hydrocarbon stream 10 such as an NGL recovery system 1,and FIG. 2 shows a simplified and first general scheme of a liquefiednatural gas plant 2 for a method for cooling an initial hydrocarbonstream 100, including the NGL recovery system 1 of FIG. 1.

An initial hydrocarbon stream 100 may be any suitable hydrocarbon streamsuch as, but not limited to, a hydrocarbon-containing gas stream able tobe cooled. One example is a natural gas stream obtained from a naturalgas or petroleum reservoir. As an alternative the natural gas stream mayalso be obtained from another source, also including a synthetic sourcesuch as a Fischer-Tropsch process.

Usually such an initial hydrocarbon stream is comprised substantially ofmethane. Preferably such an initial feed stream comprises at least 50mol % methane, more preferably at least 80 mol % methane.

The NGL recovery system 1 usually involves one or more gas/liquidseparators 14 such as distillation columns and/or scrub columns toseparate the mixed hydrocarbon feed stream 10 into at least a lightstream and one or more heavy streams at relatively low pressure, forexample in the range of 20 to 35 bar. As the mixed hydrocarbon feedstream 10 is usually provided from a high pressure initial hydrocarbonstream 100, for example in the range of 40 to 70 bar, it needs to beexpanded prior to the separator, for instance using one or moreexpanders 12.

Any form of gas/liquid separator adapted to provide at least oneoverhead stream, usually a gaseous overhead stream, and usually anoverhead stream enriched in one or more lighter hydrocarbons such asmethane, and at least one bottom stream, usually a liquid stream, andusually enriched in one or more heavier hydrocarbons, is suitable. Incertain circumstances, an overhead stream and/or a bottom stream may bea mixed phase stream.

An example of a suitable first gas/liquid separator 14 is a“demethanizer” designed to provide a methane-enriched overhead stream,and one or more C₂+ streams in the form of liquid streams at or near thebottom enriched in C₂+ hydrocarbons. However, depending on compositionof the mixed hydrocarbon feed stream and the desired specification ofthe light overhead stream, the first gas/liquid separator 14 may be ade-ethanizer, a de-propanizer, or a de-butanizer or a scrub column,instead of a de-methanizer.

The term “mixed hydrocarbon feed stream” as used herein relates to afeed stream comprising methane (C₁) and at least 5 mol % of one or morehydrocarbons selected from the group comprising: ethane (C₂), propane(C₃), butanes (C₄), and C₅+ hydrocarbons.

The terms “light” and “heavy” are defined relative to each other, andmake reference to the overhead stream respectively the bottom streamfrom the one or more gas liquid separators 14. The composition of the“light” and “heavy” hydrocarbon streams depends on the composition ofthe feed gas as well as on the design and operation conditions of thegas liquid separators.

The term “heavy hydrocarbon stream” relates to a stream comprising arelatively higher content of heavier hydrocarbons than the lightoverhead stream. For instance, the heavy hydrocarbon stream could be aC₂+ hydrocarbon stream, which predominantly comprises ethane (C₂) andheavier hydrocarbons. The relative amount of ethane is higher than therelative amount of ethane in the feed stream, but a C₂+ stream couldstill comprise some methane. Likewise, a C₃+ hydrocarbon stream, a C₄+hydrocarbon stream or a C₅+ hydrocarbon stream is relatively rich inpropane and heavier, butanes and heavier, or, respectively, pentanes andheavier.

The light overhead stream may still comprise a minor (<10 mol %) amountof C₂+ hydrocarbons (ethane and heavier), but is preferably >80 mol %,more preferably >95 mol % methane.

FIG. 2 shows an initial hydrocarbon stream 100 containing natural gas,which is cooled by a first cooling stage 104 to provide a cooled andpartly condensed initial hydrocarbon stream 110. The first cooling stage104 may comprise one or more heat exchangers either in parallel, seriesor both, in a manner known in the art. The provision of cooling to thefirst stage cooling 104 is known to the person skilled in the art.

The cooling of the initial hydrocarbon stream 100 may be part of aliquefaction process, such as a pre-cooling stage involving a propanerefrigerant circuit (not shown), or a separate process.

Cooling of the initial hydrocarbon stream 104 may involve reducing thetemperature of the initial hydrocarbon stream 104 to below −0° C., forexample, in the range −10° C. to −70° C.

The cooled initial hydrocarbon stream 110 is passed into a separatorsuch as a condensate stabilisation column 108, usually operating at anabove ambient pressure in a manner known in the art. The condensatestabilisation column 108 provides overhead a mixed hydrocarbon feedstream 10, preferably having a temperature below −0° C., and a bottomstabilized condensate steam 120. The overhead mixed hydrocarbon feedstream 10 is an enriched-methane stream compared to the cooled initialhydrocarbon stream 110.

The mixed hydrocarbon feed stream 10 comprises methane and one or moreof C₂, C₃, C₄ and C₅+ hydrocarbons. Typically, the proportion of methanein the mixed hydrocarbon feed stream 10 is 30-50 mol %, with significantfractions of ethane and propane, such as 5-10 mol % each.

In NGL recovery, it is desired to recover methane in a mixed hydrocarbonstream (for example, for use as a fuel or to be liquefied in the LNGplant 2 and provided as additional LNG), and to provide at least a C₂+stream, optionally one or more of a C₂ stream, a C₃ stream, a C₄ stream,and a C₅+ stream (not shown).

In FIGS. 1 and 2, at least a fraction, usually all, of the mixedhydrocarbon feed stream 10 passes into the NGL recovery system 1. Themixed hydrocarbon feed stream 10 passes through one or more expanders 12to provide a reduced pressure and mixed-phase (liquid and vapour)hydrocarbon stream 20, and then enters the first gas/liquid separator 14at a suitable height. The first gas/liquid separator 14 is adapted toseparate the liquid and vapour phases, so as to provide a light overheadstream 30 and a heavy bottom stream 50. The first gas/liquid separator14 may include a reboiler and a first reboiler vapour return stream (notshown) in a manner known in the art.

The nature of the streams provided by the first gas/liquid separator 14can be varied according to the size and type of separator, and itsoperating conditions and parameters, in a manner known in the art. Forthe arrangement shown in FIGS. 1 and 2, it is desired for the lightoverhead stream 30 to be methane-enriched, preferably to be >90 mol %methane.

The heavy bottom stream 50 can be >90 or >95 mol % ethane and heavierhydrocarbons, and can be subsequently fractionated or otherwise used ina manner known in the art for an NGL stream.

The light overhead stream 30 can now be recompressed by one or morefirst compressors 16 and one or more second compressors 22. For thispurpose, FIGS. 1 and 2 show one or more first compressors 16, having afirst inlet 17 and first outlet 18, and one or more second compressors22 having a second inlet 23 and a second outlet 24.

The one or more second compressors 22 are provided downstream of the oneor more first compressors 16 such that a second inlet of the one or moresecond compressors 22 can receive at least part of the first compressedlight stream from a first outlet of the one or more first compressors16. Preferably, there is no cooler present in the line between the firstoutlet of the one or more first compressors 16 and the one or moresecond compressors 22, such that the one or more second compressorsreceive an uncooled first compressed light stream.

Compression of a methane-rich gaseous stream is known in the art, andthe first and second compressors 16, 22 may comprise any knownapparatus, device or unit in one or more sections, steps or stages ableto increase the pressure on the light stream. Types and forms ofsuitable compressors and recompressors are well known in the art.

In one embodiment disclosed herein, one or more of the expanders 12prior to the first gas/liquid separator 14 are mechanically-linked toone or more of the first compressors 16. Such mechanical-linking mayoccur by any known linkage, one example of which is shared or commondriveshaft 21. The mechanical linking of an expander and a compressor,in order to use some of the work energy provided from the expander bythe expansion of a gas therethrough, to partly or fully drive amechanically linked compressor, is known in the art. FIGS. 2 and 3herewith show such a driveshaft 21, whilst the same is shown in FIG. 1in broken form for schematic layout purposes only.

In this way, operation and performance of the first compressor 16 isrelated to operation and performance of the expander 12 as discussedfurther hereinafter.

Each of the first compressor(s) 16 is able to compress at least afraction of the light overhead stream 30 to provide a first compressedlight stream 40 in a manner known in the art.

Between the first outlet 18 and first inlet 17 of each first compressor16, there is a first compressor recycle line 42 which is able to take atleast a fraction of the first compressed light stream 40 from a firstcompressor recycle stream inlet 41 and recycle it back into the path ofthe light overhead stream 30 via first compressor recycle stream outlet45. The division of the first compressed light stream 40 between a firstcompressed continuing stream 52 and a first recycle stream 42 may becarried out by any suitable divider or stream splitter known in the art.The division of the first compressed light stream may be anywherebetween 0-100% for each of the continuing stream 52 and first recyclestream 42 as discussed further hereinafter.

The first compressor recycle line 42 is a dedicated line around thefirst compressor 16 and preferably only includes one or more controlvalves 44 required to change the pressure of the first compressorrecycle stream 42 to approximate or equate its pressure to the intendedpressure of the C₂ overhead stream 30 for the suction side of the firstcompressor 16. In particular, it is noted that there is no cooler orcoolers on the first compressor recycle line 42 (adapted to change thetemperature of the first compressor recycle stream 42, generallydownwardly, whilst the pressure of the first compressor recycle stream42 is wholly or substantially unchanged). Thus, the CAPEX and OPEX ofneeding one or more coolers is avoided, whilst the first compressorrecycle line 42 still provides anti-surge control around the firstcompressor 16.

In this way, the first compressor recycle stream 42 is uncooled, and/orthe first compressor recycle line 42 is an uncooled recycle line.

The first compressed continuing stream 52, being some or all of thefirst compressed light stream 40, may then pass through an optional oneor more throttle control valves 26, and then pass as a second compressorfeed stream 54 into the one or more second compressors 22, each secondcompressor 22, to provide one or more further compressed light streams60 in a manner known in the art. The or each second compressor 22 may bethe same or similar to a ‘boost’ compressor, generally having adedicated driver or drive mechanism separate from the one or more firstcompressors 16.

Around the or each second compressor 22 is a second compressor recycleline 32, such that the one or more further compressed light streams 60can be divided by a divider or stream splitter known in the art,anywhere between 0-100%, between a final compressed stream 70 and asecond compressor recycle stream 32. The second compressor recyclestream 32 has a second compressor recycle stream inlet 33. The secondcompressor recycle stream 32 includes one or more coolers 34, preferablyone or more water and/or air coolers, known in the art and adapted toreduce the temperature of the second compressor recycle stream 32. Theone or more air coolers 34 are followed by one or more control valves 36to provide a final recycle stream 38 for re-injection into the mainlight stream in advance of the second inlet 23 of the second compressor22 at second compressor recycle stream outlet 39.

The second compressor recycle line 32 provides anti-surge control aroundthe second compressor 22 in a manner known in the art. The secondcompressor recycle line 32 is a dedicated line around the secondcompressor 22. In particular, it is noted that the one or more coolers34 are only required to cool the percentage of the further compressedlight stream 60 which is passed into the second compressor recycle line32, which percentage is commonly zero or minimal, thus minimising theOPEX of the one or more coolers 34.

FIGS. 1 and 2 show a simplified arrangement of the recompression of alight stream using a first compressor 16 which has a dedicated firstcompressor recycle line 42 that does not require dedicated or externalcooling, and a second compressor 22 with a dedicated second compressorrecycle line 32. Thus, the first and second compressor recycle lines 32,42 are independent, and can be independently controlled.

FIGS. 1 and 2 also show a first bypass line 80 around the expander 12having a control valve 82. In this way, at least a fraction, optionallyall, of a mixed hydrocarbon feed stream 10 not requiring to be passedthrough the expander 12 can pass through the first bypass line 80 toprovide the mixed-phase hydrocarbon stream 20. This arrangement mayoccur during start up of the NGL recovery system 1, and/or duringtripping of one or more of the expanders 12 as further discussedhereinafter.

Similarly, FIGS. 1 and 2 also show a second bypass line 90 with aone-way valve 92 around the first compressor 16 so as to take at least afraction, optionally all, of the light overhead stream 30 around the oreach first compressor 16 to provide the second compressor feed stream 54for the or each second compressor 22. The second bypass line 90 may beused during start-up of the NGL recovery system 1, especially wherethere is no driving power for first compressor 16, (which can bemechanically linked to and therefore driven by the expander 12). Thesecond bypass line 90 may also be useful where one or more of the firstcompressors 16 ‘trips’ as further discussed hereinafter.

As shown in FIG. 2, the final compressed stream 70 may be wholly orpartly used as fuel gas 72, or passed to a gas network e.g. to providedomestic gas, or subsequently cooled, preferably liquefied, to provide acooled hydrocarbon stream such as LNG. The cooling and preferredliquefaction may be carried out after passage along line 71. The coolingand preferred liquefaction occurs in the second cooling stage 112,typically comprising one or more heat exchangers, to provide a liquefiedhydrocarbon stream 130, as shown in FIG. 2. Suitable liquefactionprocesses for such second cooling stages are known to the person skilledin the art and will not be further described here.

FIG. 3 shows a simplified and second general scheme of a liquefiednatural gas plant 2 for a method for cooling an initial hydrocarbonstream 100 as described for FIG. 2, further including NGL recoveryscheme 3 based on having a first expander and first compressor string A,and a second expander and first compressor string B.

In FIG. 3, a mixed hydrocarbon feed stream 10 is divided by a streamsplitter 11 into at least two, preferably two or three, part-feedstreams 10 a and 10 b, which pass into respective expanders 12 a and 12b which are mechanically linked by respective common driveshafts 21 aand 21 b to respective first compressors 16 a and 16 b. The division ofthe mixed hydrocarbon feed stream into the part-feed streams 10 a and 10b may be any ratio or percentage, but will generally be equal duringnormal and conventional operation of the second NGL recovery stream 3wherein the expanders 12 a and 12 b have the same capacity. Variationsin the size, type, capacity, number and their balance of the expanders12, and in consequence in the size, capacity, type, number and balanceof the first compressors 16, are known to the skilled man in the artwith knowledge of NGL recovery processes, operations and parameters.

Each expander 12 a, 12 b provides a mixed-phase hydrocarbon stream 20 a,20 b respectively, which can be combined by a suitable combiner 15 suchas a T-piece, to provide a single mixed-phase hydrocarbon stream 20 topass into the first gas/liquid separator 14 as hereinabove described.Optionally, one or more of the mixed-phase hydrocarbon streams 20 a and20 b may pass directly into the first gas/liquid separator 14 withoutcombination with the or all of the other mixed-phase hydrocarbonstreams.

The first gas/liquid separator 14 provides a light overhead stream 30,and a heavy bottom stream 50 as hereinbefore described. The lightoverhead stream 30 can then be divided by a stream splitter 31 in amanner known in the art, to provide at least two, preferably two orthree, part-light streams 30 a, 30 b which pass respectively into thetwo first compressors 16 a, 16 b through their first inlets 17 a, 17 bto provide two respective first compressed light streams 40 a, 40 b atfirst outlets 17 b, 18 b. 0-100% of the first compressed light streams40 a, 40 b may pass into two respective first compressor recycle lines42 a, 42 b through first compressor recycle inlets 41 a, 41 b forrecycle through respective control valves 44 a, 44 b and return to thesuction sides of the two first compressors 16 a, 16 b via firstcompressor recycle outlets 45 a, 45 b as described hereinabove.

That fraction of each of the first compressed light streams 40 a and 40b not passing into the first compressor recycle lines 42 a, 42 b providefirst compressed continuing streams 52 a, 52 b, which can pass throughrespective throttle control valves 26 a, 26 b before being combined by acombiner 53 to provide a second compressor feed stream 54 which passesto a second compressor 22 through an inlet 23, and out through an outlet24 as a further compressed light stream 60. As described above, afraction between 0-100% of the further compressed light stream 60 canprovide a second compressor recycle stream 32 via a second compressorrecycle inlet 33 and second compressor recycle outlet 39, whilst a finalcompressed stream 70 can be used as described above, for example as oneor more other fuel stream, export stream, or for cooling, preferablyliquefying, to provide a liquefied hydrocarbon stream such as LNG.

The combination of the first expander 12 a, the mechanically linkedfirst compressor 16 a, and their associated lines, provide the firststring A, whilst the combination of the second expander 12 b, themechanically linked first compressor 16 b, and its associated lines,provide the second string B.

In this way, the user of the second NGL recovery scheme 3 is able tohave greater options and flexibility concerning the flow of the mixedhydrocarbon feed stream 10 through the second NGL recovery scheme 3, inparticular operations and flows through the expanders 12 and firstcompressors 16. As well as providing operational advantages duringnormal and/or conventional running of an NGL recovery scheme, thisarrangement further provides two further advantages.

Firstly, should any string of a multi-string NGL recovery scheme not beable to run normally, either by accident or design, the continuance ofthe NGL recovery is possible through one or more of the other strings.In particular, where a string should ‘trip’, then the or each otherstring is able to continue operation of the NGL recovery, even if thevolume and/or mass of the mixed hydrocarbon feed stream continues at thesame level, or continues at a significant level.

The ‘tripping’ of a expander-compressor string can occur for a number ofreasons, and/or in a number of situations. Common examples include‘overspeed’, for instance where the driver produces more power than thatrequired by the compressor and ‘vibration’ when the compressor isoperating beyond the flow envelope and the flow angle with respect tothe vane angle is incorrect.

A second particular advantage of the second NGL recovery scheme 3 shownin FIG. 3 is during start-up of the recovery scheme. By providing two ormore strings, each string can be separately started at a different time,and optionally with different starting parameters than each otherstrings. Thus, the user has greater options and control over thestart-up of all the strings prior to full and normal operation of theoverall second NGL recovery scheme 3.

As an example, at the start-up of an NGL recovery scheme, the mixedhydrocarbon feed stream 10 is usually passed through a first bypassstream 80 to bypass the first expanders 12 a, 12 b to provide themixed-phase hydrocarbon stream 20 because the pressure in the mixedhydrocarbon stream 10 may already be at a low level, such that expansionin first expanders 12 a, 12 b is unnecessary, or would result in too lowa pressure in mixed-phase hydrocarbon stream 20. Bypassing the firstexpanders 12 a, 12 b provides a higher pressure in light overhead stream30 than would otherwise occur.

Similarly, the light overhead stream 30 can pass through the secondbypass line 90, and one-way valve 92 to bypass the first compressors 16a, 16 b, especially where these are not provided with power or otherwisedriven by the first expanders 12 a and 12 b which are being similarlyby-passed.

It is a particular advantage of the method and apparatus disclosedherein that through pressure and flow control of each bypass stream andeach part-stream, as the flow and/or pressure of the mixed-phasehydrocarbon stream 10 increases during start-up, one or more strings ofa multi-string NGL recovery scheme can be separately started and broughtup to normal operation as a controlled procedure. Thus, the two throttlecontrol valves 26 a, 26 b in the paths of the first compressorcontinuing streams 52 a, 52 b, allow control of the introduction oflight overhead streams 30 a, 30 b into the first compressors 16 a, 16 bin calculation with reduction of the flow of the second bypass stream90. The two throttle valves 26 a, 26 b can control the pressure at thedischarge of each of the first compressors 16 a, 16 b, especially nearstonewall of each first compressor 16 a, 16 b, which most usually canoccur during start-up and following any tripping of a string.

In this way, the pressure of the light stream in the second bypass line90 does not hinder the start-up of each of the first compressors 16 a,16 b, either together or independently. This arrangement seeks to ensuremaximum forward flow through the or each first compressor, (and hence nooverheating), without operating in the stonewall region.

It is a further advantage of a multi-string NGL recovery scheme that oneor more of the first compressors 16 a, 16 b can be isolated from the oreach other first compressors, so as to reduce interaction between thefirst compressors 16 a, 16 b.

A person skilled in the art will readily understand that the presentinvention may be modified in many ways without departing from the scopeof the appended claims.

1. Apparatus for treating a mixed hydrocarbon feed stream, the apparatusat least comprising an NGL recovery system comprising: (a) an expanderto expand the mixed hydrocarbon feed stream into a mixed-phasehydrocarbon stream; (b) a first gas/liquid separator to separate themixed-phase hydrocarbon stream to provide at least a light overheadstream and a heavy bottom stream; (c) a first compressor having a firstinlet and a first outlet, to compress the light overhead stream and toprovide a first compressed light stream; (d) a second compressor havinga second inlet and a second outlet, to compress the first compressedlight stream to provide a further compressed light stream; (e) a firstcompressor recycle line around the first compressor between the firstoutlet and the first inlet; and (f) a second compressor recycle lineprovided with one or more in-line coolers around the second compressorbetween the second outlet and the second inlet.
 2. The apparatus asclaimed in claim 1, wherein the expander is one of two or more expandersarranged in parallel, and the first compressor is one of two or morefirst compressors arranged in parallel, whereby the second compressor isarranged to receive the combined first compressed light streams from thetwo or more first compressors.
 3. The apparatus as claimed in claim 1,wherein the expander and the first compressor are mechanically linked bya common drive shaft.
 4. The apparatus as claimed in claim 1, whereinthe gas/liquid separator is a distillation column.
 5. The apparatus asclaimed in claim 1, further comprising one or more cooling stagesupstream and/or downstream of the NGL recovery system.
 6. The apparatusas claimed in claim 5, wherein the one or more cooling stages comprise afirst cooling stage able to cool an initial hydrocarbon stream toprovide a cooled and partly condensed initial hydrocarbon stream; andfurther comprising a separator able to separate the cooled and partlycondensed initial hydrocarbon stream to provide the mixed hydrocarbonfeed stream and a stabilized condensate stream.
 7. The apparatus asclaimed in claim 5, wherein the one or more cooling stages comprise asecond cooling stage downstream of the second compressor arranged toreceive and further cool at least a fraction of the further compressedlight stream to provide a cooled hydrocarbon stream.
 8. The apparatus asclaimed in claim 1, further comprising a throttle control valve betweenthe first compressor and the second compressor, to throttle the pressurebetween the first compressor and the second compressor.
 9. The apparatusas claimed in claim 1, further comprising a first bypass line around theexpander and a second bypass line around the first compressor.
 10. Theapparatus as claimed in claim 1, wherein the first compressor recycleline is an uncooled recycle line.
 11. The apparatus as claimed in claim1, wherein no cooler is present between the first outlet and the secondinlet.
 12. A method of treating a mixed hydrocarbon feed stream, themethod at least comprising the steps of: (a) providing a mixedhydrocarbon feed stream; (b) expanding the mixed hydrocarbon feed streamthrough an expander to provide a mixed-phase hydrocarbon stream; (c)passing the mixed-phase hydrocarbon stream into a first gas/liquidseparator to provide at least a light overhead stream and a heavy bottomstream; (d) passing the light overhead stream through a first compressorhaving a first inlet and a first outlet, to provide a first compressedlight stream; (e) passing the first compressed light stream through asecond compressor having a second inlet and a second outlet, to providea further compressed light stream; and (f) selectively recycling atleast a fraction of the first compressed light stream through a firstcompressor recycle line, around the first compressor between the firstoutlet and the first inlet; and (g) selectively cooling and recycling atleast a fraction of the further compressed light stream through a secondcompressor recycle around the second compressor between the secondoutlet and the second inlet, the second compressor recycle line providedwith one or more in-line coolers.
 13. The method as claimed in claim 12,wherein the mixed hydrocarbon feed is derived from a natural gas stream.14. The method as claimed in claim 12, wherein at least a fraction ofthe further compressed light stream is subsequently cooled to provide acooled hydrocarbon stream.
 15. The method as claimed in claim 14,wherein the subsequent cooling comprises liquefying, to provide thecooled hydrocarbon stream in the form of a liquefied hydrocarbon stream.16. The method as claimed in claim 12, further comprising cooling aninitial hydrocarbon stream to provide a cooled and partly condensedinitial hydrocarbon stream; and separating the cooled and partlycondensed initial hydrocarbon stream to provide the mixed hydrocarbonfeed stream and a stabilized condensate stream.
 17. The method asclaimed in claim 12, wherein the selective recycling in step (f) isuncooled.
 18. The method as claimed in claim 12, wherein the firstcompressed light stream is passed to the second compressor withoutcooling.
 19. A method of cooling an initial hydrocarbon stream,comprising at least the steps of: (i) passing the initial hydrocarbonstream through a separator to provide a stabilized condensate stream anda mixed hydrocarbon feed stream; (ii) treating the mixed hydrocarbonfeed stream to provide one or more further compressed light streams; and(iii) cooling at least a fraction of the one or more further compressedlight streams to provide a cooled hydrocarbon stream, wherein saidtreating in step (ii) at least comprises the steps of: (b) expanding themixed hydrocarbon feed stream through an expander to provide amixed-phase hydrocarbon stream; (c) passing the mixed-phase hydrocarbonstream into a first gas/liquid separator to provide at least a lightoverhead stream and a heavy bottom stream; (d) passing the lightoverhead stream through a first compressor having a first inlet and afirst outlet, to provide a first compressed light stream; (e) passingthe first compressed light stream through a second compressor having asecond inlet and a second outlet, to provide the further compressedlight stream; and (f) selectively recycling at least a fraction of thefirst compressed light stream through a first compressor recycle line,around the first compressor between the first outlet and the firstinlet; and (g) selectively cooling and recycling at least a fraction ofthe further compressed light stream through a second compressor recyclearound the second compressor between the second outlet and the secondinlet, the second compressor recycle line provided with one or morein-line coolers.
 20. The according to claim 19, wherein the coolingcomprises liquefying to provided the cooled hydrocarbon stream in theform of a liquefied hydrocarbon stream.